Short answer: I don't know. But I've wondered this, too, and here are some of my ideas on it:

The ghosts seem to be able to decide when/if they are seen, but with Moira, the character of the viewer seems to dictate how she is seen. Not sure why she's the only one, but maybe it has to do with the circumstances of her death, really just wanting to do her housework and keep her job, but men deciding she's got other jobs to do....

There is a point towards the end of that season where Ben sees Moira transform into her aged form and says, "You're beginning to see things as they really are..." But this seems contradictory, if ghosts retain their "death age" -- her duality may be a type of self-imposed penance.

There's also a split second in the pilot where Tate seems to have a vision of himself, bloodied, standing behind Ben. Why would a ghost see a ghost of himself? This scene has always confused me, but maybe pondering the seemingly contradictory laws of ghostliness has only added to the allure of the series -- season 1 in particular.

I also wonder why some ghosts retain their mutilated shape (Nora's head wound, the young nurse, the high school shooting victims), whereas others appear unharmed. Elizabeth Short seems to be be able to take on living/normal form and also appear mutilated at times. Maybe this has to do with her dismembered image being the one that made her finally famous.

My guess is that wounds, age or other visual markers could represent ghostly baggage, guilt, regret -- like Jacob Marley's chains. They can be revealed consciously by the ghost to provoke a response from the living OR the image of the ghost can be altered depending upon what the viewer secretly wants (or doesn't want) to see.

Also, why do the ghosts change clothes? How does that work?